Among the three most versatile commodity plastics, which are polyethylene (PE), polystyrene (PS) and polypropylene (PP), polypropylene is considered to possess the most favorable properties profile of the three for a variety of applications. These applications include, for example, oriented and non-oriented films, textile fibers, nonwovens and a variety of injection molded parts. Comparing the properties, it is well known that polypropylene has a higher modulus and heat deflection temperature (HDT) than polyethylene. The higher the modulus and HDT, the more suited the polymer is for durable applications in the appliance and automotive segments. Additionally, because polypropylene is nonpolar, it resists degradation by common solvents, such as acids and alkalis. Compared to polystyrene, polypropylene is preferred in applications requiring good organoleptic performance, high barrier properties and the living hinge property. Finally, polypropylene blends well with a variety of other polymers, and in impact-modified form occupies a dominant position in the automotive industry in the areas of bumpers, side panels, floor mats, dashboards and instrument panels.
However, there exist some polymer applications where polypropylene is not the preferred plastic of choice. Examples of such polymer application areas include thermoforming and foaming. Foamed polymers find usage in automotive, marine, appliance and packaging applications because of their good insulating and structural properties at low added weight. Thermoforming is a popular fabricating mode that competes favorably with injection molding in the making of thin-walled containers. Polypropylene's deficiencies in foaming and thermoforming are believed to be related to its generally poor melt strength and rapid melt viscosity drop, poor sheet sag and comparatively slow crystallization kinetics. For example, to successfully foam an article formed from a polyolefin, it is desirable that the polyolefin selected for foaming possess high melt strength. With high melt strength, the bubble growth rate within the polyolefin can be controlled without premature bursting. Controlling bubble growth rate is also important for ensuring a uniform distribution of cell sizes, which leads to greater product uniformity. Additionally, broader polymer processing temperature windows are desirable so that when the polymer is used in an article forming process, the temperature variances along the process line are less disruptive to the fabrication of a quality product.
So that manufactures of plastic articles and the consuming public may more fully benefit from the use of polypropylene in a broader array of applications, further development and investigation is needed in the area of polypropylene compositions and methods of manufacturing. This is particularly so, as described above, when the article manufacturing process requires that the polymer be foamed.